Well to give some context inside and outside of this incident.

Let's start with this incident, the term "sb" is common used in china, not only throughout our daily school lives, and it's even more common on the internet. They are some spoiled kids (there are lots of them, because single child policy) who take serious offence to the use of sb, which has happened to me on a personal level in the NA server when I said this to a fellow Chinese person as a joke. Point is, "sb" is not really all that offensive, it's basically like idiot, dumbass, stupid in literal translation, most of its use is not meant to offend, and it's used in basically all sort of contexts.

Here is some background on the hate. From China's hate against Korea started when Korea first started releasing their TV series into China, which got a lot of women hooked. Making a lot of men annoyed at the source, obviously at the time, this is not at all a big deal, it's basically like the western hate for Justin Bieber or the movie Twilight. How it became a big deal is when Korea made claims that many significant figures in China were Korean. It was at first backed by evidence, but eventually it became ludicrous. They might as well claim every single Chinese were born of Korea origin. A notable example would be, Confucius, who is a famous Chinese philosopher. As some of you may know, China is filled with national pride, so many people took strong offence to such claims, and dismissed it as Korean being dumb. Around that time, the racist term which is literally translates into "bat" is used to describe Koreans. To be fair, I do not know Korea's side of the story. What I do know from my time of studying world history as a fact, is that, Korea through its history was often a vassal of China, meaning self governed, but technically sworn loyalty to a bigger kingdom, and is bound to sending gifts and tributes every single year, or cycle. There are still many things I do not know, especially from the Korean perspective. I hope this will not be taken as racism, I'm only sharing what I know about the situation. If anyone has anything to add to this, feel free.

Tl;dr, this is just a solo q jab using the most commonly used word on the internet in china, hardly newsworthy. Then I gave a brief background of some china and korea hostility in recent years (honestly it's not serious, it's a joke compared to how much korea and china hates japan)